The English Abstract of Japanese Patent Application no. 2-159727 discloses a semiconductor device with a bipolar transistor of the kind mentioned in the opening paragraph in which the collector, base, and emitter zones are connected to contact regions situated adjacent the strip and provided in the layer of semiconductor material present on the substrate.
In the manufacture of this known bipolar transistor, the strip of insulating material is first provided on the layer of semiconductor material. The portion of the layer of semiconductor material situated below the strip forms the collector zone of the transistor. Then the base zone is formed in that a p-type dopant is provided at one side of the strip, and in that subsequently a heat treatment is carried out whereby p-type dopant diffuses into the portion of the layer of semiconductor material situated below the strip, forming a base-collector junction them. Then three strip-shaped regions are formed in the layer of semiconductor material at the same side of the strip, the layer of semiconductor material being removed between the strip-shaped regions up to the strip of insulating material. The outermost regions are provided with an extra quantity of p-type dopant, whereby the two base contact regions are formed. Then an n-type dopant is provided in the central region and a heat treatment is carded out. N-type dopant diffuses as a result from the central region into the base zone. The emitter zone and an emitter-base junction are thus formed in the base zone. The central n-type doped region itself forms the emitter contact region.
The transistor thus formed has an intrinsic base formed by that portion of the base zone which is situated between the base-collector junction and the emitter-base junction. Between the intrinsic base and the contact regions, the base zone forms a narrow, extrinsic base situated below the strip of insulating material. The extrinsic base forms an undesired resistance. This resistance is as small as possible when the extrinsic base is as small as possible. This is the case when the mutual interspacings between the three contact regions are as small as possible.
The three strip-shaped regions situated at one side of the strip of insulating material are formed in the layer of semiconductor material by means of a photolithographic process in the known bipolar transistor. After a photoresist mask has been provided, defining these regions, portions of the layer of semiconductor material not covered by this mask are etched away. It is true for a photolithographic process that details smaller than a critical dimension valid for that process cannot be pictured in the photoresist. The three contact regions, accordingly, cannot be realised with interspacings smaller than the critical dimension. In a practical, modem lithographic process, this critical dimension is, for example, 0.5 .mu.m.